Interest in medical prosthetic implants with longer lifespans has increased due to demographic shifts in the population of medical implant recipients. As in the general population, the life expectancy of medical implant recipients has been steadily increasing. Additionally, the number of younger and more active implant recipients has also been increasing. Each of these trends highlights the importance of longer-lasting, high performance orthopedic implants that are less likely to require replacement, i.e., revision, during a recipient's lifetime.
Thus, of concern is that such longer-lasting implants be able to withstand years of wear and the variety of stresses associated with being implanted in a body. Joint replacement implants must be strong enough to tolerate changing mechanical loads, while at the same time, transferring portions of the loads to surrounding bone tissue. Another requirement is that the bearing (i.e., articulating) surfaces of joint replacement implants must be resistant to a corrosive environment and withstand body and surface contact forces and adhesive and abrasive wear processes that are associated with movement of the joint. Mechanical and physical properties suitable for withstanding years of mechanical loading and wear can be difficult to find in a single material.
Currently, metals, including metal alloys, are used as structural material for medical implants. Metal alloys such as steel, cobalt alloys, and titanium alloys exhibit a range of strengths, hardness, and fatigue resistance. Moreover, these metals can be formed by metalworking processes and machined. Commonly, implants include a metal bearing surface configured to move against a second bearing surface composed of a low-friction polymer, such as ultra high molecular weight polyethylene (UMWHPE), or another metal component. Particles and debris have been known to dissociate from these implants due to wear processes once implanted in a recipient's body. These dissociated particles are problematic, as those that become disposed in surrounding tissue have been linked to inflammation and degradation of surrounding tissue. This inflammatory reaction can lead to joint pain and loosening of the implant. Eventually, the condition can require removal and replacement of the implant in a complicated and troublesome revision procedure. Hard particles such as cement particles (in devices implanted with cement) or delaminated metal from an implant's porous or textured fixation surface can become lodged in the articulating surface of a polymeric implant or trapped between opposing articulating surfaces and scratch the previously smooth metallic surface, and thereby reduce implant performance, increase abrasive wear of the articulating counterface, and even lead to delamination of other surface material.
Previous efforts to reduce the wear of metallic implant surfaces have included surface modifications such as ion implantation, gas nitriding, and high temperature oxidation. Each approach can have limitations. For example, generally, the peak hardness of surfaces obtained by ion implantation and nitriding is not as high as the peak hardness that can be obtained by other surface modifications such as ceramic overlay coatings. Ion implantation approaches can also have theoretical or economic limits on the depth of the surface modified material. Substrates underlying surface layers created by high temperature oxidation (e.g., Zr-2.5 Nb alloy) are not as strong or hard as other substrate metals (e.g., CoCrMo or Titanium alloys) and may not be compatible with conventional processes for creating porous tissue ingrowth surface structures.
In a different approach, bulk metal substrates in implants have been replaced with bulk ceramics, such as in certain hip implants that include bulk oxide ceramic femoral heads. Such ceramics do not corrode in the body, are wear-resistant, and they can withstand large compressive loads. However, bulk ceramics tend to be stiffer and more brittle than metals. Reports have shown that some bulk ceramic implant components are prone to catastrophic fracture and, thus, may require immediate revision.
To obtain the beneficial wear properties of bulk ceramics without the risk of fracture, there have been attempts to apply ceramic coatings to the articulating surface of a metal implant. The properties of these ceramic coatings are quite varied, depending on the ceramic type and manner of application. However, the development of implants coated with particular ceramics (e.g., aluminum oxide) has been limited. Techniques for applying particular ceramic coatings have resulted in coatings that are alternately too thin, porous, unstable in the body, or poorly adhered to the underlying metal substrate for use on orthopaedic bearing surfaces.